In addition to the hydrorefining state-of-the-art practiced in the presence of a hydrorefining catalyst, hydrogen and high temperatures and pressures, other techniques have been disclosed for the removal of these nitrogen compounds. Recently, two U.S. Pat. Nos. 4,332,676 and 4,332,675 issued to Baset, which disclose a process for the removal of basic nitrogen compounds from organic streams inclusive of petroleum oils utilizing gaseous sulfur dioxide to thereby precipitate a salt comprising the basic nitrogen compound, sulphur dioxide and water with downstream separation of the precipitated salt. Both of these patents concern a single-phase treatment system with the content of water in the separation system in '675 being substantially eliminated and the quantity of water in '675 being such that only a single phase system is existent. In fact, in the latter reference the addition of water is limited to a concentration only to the extent that a two-phase liquid system will never be formed. It is also disclosed that a non-polar solvent can be utilized in the contacting step such as a petroleum ether, a lower paraffinic hydrocarbon or an aromatic hydrocarbon such as toluene. While the types of basic organic nitrogen compounds extracted in the instant invention are either similar to or the same as those described in column 2 of the '676 disclosure, the means by which the process is undertaken in the instant invention is very different from that disclosure.
In the October 1983 issue of Chemical Engineering an article by Desai and Madgavkar, recognizes a method to remove catalyst-poisoning nitrogen compounds from shale oil by solvent extraction with a formic acid/water solvent prior to hydrotreating. The advantage of this technique is a lowering of the hydrogen consumption and a reduction of the nitrogen content to a tolerable level feasible for downstream processing of the shale oil. It should be noted that the nitrogen compounds indigenous to the shale oil are unique and will not necessarily behave in the same manner as the nitrogen compounds indigenous to petroleum oils. Further, shale oil liquids are derived from a polymeric material, "kerogen", which is thermally decomposed into liquids which contain the nitrogen molecules. Petroleum oils are formed by biological and chemical action of nature over a much longer period of time, are more mature than shale-derived oils and have a chemical constituency far different from shale-derived oils. Also, the starting materials in formulation of the petroleum oil versus the shale oil are very different and produce a lower and different content of nitrogen compounds for the petroleum oil than the shale oil. The method of nitrogen extraction in regard to the latter can simply not be extrapolated to the former.
The addition of inorganic acids to petroleum oils to reduce the quantity of nitrogen compounds has long been established. For example, in U.S. Pat. No. 2,352,236 anhydrous hydrogen chloride is added to improve a charge stock for catalytic cracking. A dilute acid, such as sulfuric acid, is disclosed in U.S. Pat. No. 1,686,136 to complex nitrogen compounds existent in a California-derived crude oil. Organic carboxylic acids, sometimes referred to as low molecular weight fatty acids of high volatility, have been used to complex nitrogen-bases in such disclosures as U.S. Pat. Nos. 2,263,175 and 2,263,176. While these latter two references employ a portion of the chemical mechanism utilized in the first step of this two-step nitrogen extraction process, they fail to disclose, suggest or even hint at the use of a second step to hydrotreat the recovered petroleum oil fraction to more precisely lower the content of the heterocyclic nitrogen compounds. Also, these references fail to teach the use of a combination carboxylic acid extraction step with such acids as an admixture of formic and acetic acids. This is important in light of the cross production of an acetic acid, i.e., formic acid will usually be present as an impurity. Thus, it may be economic and advantageous to use a mixture of such co-produced carboxylic acids as the extractant of the first extraction step.
A patent issued to Johnson et al, U.S. Pat. No. 4,409,092 in 1983, teaches formation of a high nitrogen fraction and a low nitrogen fraction, which is then subjected to phosphoric acid extraction. The fraction high in nitrogen content is catalytically cracked and then either hydrotreated or sent to phosphoric acid extraction. There is no disclosure by Johnson et al of a process whereby extraction of a petroleum oil is made in the presence of a C.sub.1 to C.sub.15 carboxylic acid extraction agent and then subsequent hydrotreatment. The patent teaches at column 14 that use of acetic acid is not desirable since such use would result in esterification of the materials being treated.
A shale oil feedstock is treated in a patent issued to Kuk et al, U.S. Pat. No. 4,483,763 in 1984. This is not a petroleum crude oil process and the nitrogen components indigenous to the shale oil are different from the nitrogen compounds of petroleum oil as taught by above-discussed Johnson et al (see column 1, line 35.sup.+). Kuk et al hydrotreat prior to division into a nitrogen lean and a nitrogen rich stream. After a hydrotreating step, which is necessary to eliminate the more easily hydrogenatable components, the intractable nitrogen components are then subject to solvent extraction. The extractant component utilized in Kuk et al is an organic polar solvent such as an alkanol. This is an active and mandatory ingredient in the Kuk et al extraction as demonstrated by Examples 8-10 (col. 5) where no carboxylic acid is present yet a reduction in nitrogen content is realized. The specific example of this reference discloses that the feed material contains 2.05 percent nitrogen. The segregated middle distillate cut contains only 0.53 percent nitrogen (a smaller amount of nitrogen compounds), which is subjected to solvent extraction.